Statistical study of extreme-ultraviolet nanoflares in the quiet-Sun transition region

Abstract

Aims. We carried out a large statistical study of ubiquitous small-scale extreme-ultraviolet (EUV) brightenings in the nanoflare energy range in the quiet-Sun transition region to derive their properties, estimate their contribution to the heating of the solar atmosphere, and compare their numbers to the coronal events published in the literature. This is the first study of this magnitude at temperatures of about 2 × 105 K. Methods. We applied a numerical method for detecting small-scale transient events in long 1D image time series. We used data recorded by the SOHO Coronal Diagnostic Spectrometer (CDS) in the transition region line O V 62.97 nm (220 000 K) and analysed 702 h of sit-and-stare time series obtained with a cadence of 15.6 s and 50 h with a cadence of 20.5 s in different quiet-Sun areas at a fixed slit position. These data span from 1996 to 2011. This analysis used a different method and a vastly larger number of data than the previous high-cadence CDS study of small events. Results. We derive histograms of event durations, of the rise and decay time, of the peak intensity and thermal energy, and we obtain a continuous spectrum of their distributions for 117 000 events, spanning the nanoflare energy range with a linear spatial extent of 2−10 arcsec and with durations between 45 s and 40 min. The event peak intensity varied by a factor of 60. We demonstrated that all categories of small-scale events in the transition region are part of a continuum of activity. We obtain a total event rate of 460 s−1 on the entire surface of the Sun. This is more than four times greater than the coronal rate. The maximum value of the duration distribution occurs at 235 s, which is twice the duration of the coronal events. The decay time and rise time difference seen from the shortest to the longest events is symmetrical. We find two event populations: the power law of the smallest events that are confined to one pixel is far steeper for the peak count rates (index of −4.1) and thermal energy (index of −7) than the power law for combined larger events that extend over two or more pixels along the slit (thermal energy power-law index from −2.1 to −3.4). Conclusions. The power law of the thermal energy of the smallest events, extrapolated to lower energies (picoflares), may provide a huge amount of energy for heating the entire transition region plasma at temperatures of about 220 000 K. An extrapolation of only the flatter power law of the larger events can also account for the entire observed emission.